Scilab Function
Last update : 8/6/2005

fchamp - direction field of a 2D first order ODE

### Calling Sequence

  `fchamp(f,t,xr,yr,[arfact,rect,strf])`
  `fchamp(f,t,xr,yr,<opt_args>)`

### Parameters

- `f`: An external (function or character string) or a list which describes the ODE.
  - `-`It can be a function name `f`, where `f` is supposed to be a function of type `y=f(t,xy [p1,..pn])`. `f` returns a column vector of size 2, `y`, which gives the value of the direction field `f` at point `xy=[x,y]` and at time `t`.
  - `-`It can also be an object of type list, `list(f,P1,..Pn)` where `f` is a function of type `y=f(t,xy,p1,..pn)` and `Pi` gives the value of the parameter `pi`.
- `t`: The selected time.
- `xr,yr`: Two row vectors of size n1 and n2 which define the grid on which the direction field is computed.
- `<opt_args>`: This represents a sequence of statements `key1=value1, key2=value2`,... where `key1`, `key2,...` can be one of the following: arfact, rect, strf (see below).
- `arfact,rect,strf`: Optional arguments, see `champ`.

### Description

`fchamp` is used to draw the direction field of a 2D first order ODE defined by the external function `f`. Note that if the ODE is autonomous, argument `t` is useless, but it must be given.

Enter the command `fchamp()` to see a demo.

### Examples

    deff("[xdot] = derpol(t,x)",..
            ["xd1 = x(2)";..
             "xd2 = -x(1) + (1 - x(1)**2)*x(2)";..
             "xdot = [ xd1 ; xd2 ]"])
    xf= -1:0.1:1;
    yf= -1:0.1:1;
    fchamp(derpol,0,xf,yf)
    clf()
    fchamp(derpol,0,xf,yf,1,[-2,-2,2,2],"011")

### See Also

`champ`, `champ1`,

### Author

J.Ph.C.